Shower Tap Diverter Valve

[diynewbie]

Hi all, DIY rookie and have an issue with malfunctioning diverter valve - water currently flows through both tap and shower head regardless of diverter position. Had a look at the valve today and it appears that the upper thread just above the rubber washer is broken. Unsure of make/model of tap and was wondering if anyone had any helpful suggestions. No obvious damage to washer and have given it a proper clean, reattached etc and same issue. Feel like I probably just new a new valve but keen to explore all options as think it would be hard to source something similar and unkeen to change entire taps as the panelling on bath is a bit of a nightmare. Any help would be great. Cheers.